Method for manufacturing a lithographic printing plate precursor having a patterned back layer

ABSTRACT

A method for manufacturing a lithographic printing plate precursor includes the steps of providing a support as a web, coating an image recording layer on the front side of the support, and depositing a back layer on the back side of the support using a deposition technique which is capable of depositing the back layer according to a predefined image. The method enables stacking and recutting of lithographic printing plate precursors without the need for interleafs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2014/062540, filed Jun. 16, 2014. This application claims thebenefit of European Application No. 13172426.2, filed Jun. 18, 2013,which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic printing plateprecursor.

2. Description of the Related Art

Lithographic printing typically involves the use of a so-called printingmaster such as a printing plate which is mounted on a cylinder of arotary printing press. The master carries a lithographic image on itssurface and a print is obtained by applying ink to said image and thentransferring the ink from the master onto a receiver material, which istypically paper. In conventional lithographic printing, ink as well asan aqueous fountain solution (also called dampening liquid) are suppliedto the lithographic image which consists of oleophilic (or hydrophobic,i.e. ink-accepting, water-repelling) areas as well as hydrophilic (oroleophobic, i.e. water-accepting, ink-repelling) areas. In so-calleddriographic printing, the lithographic image consists of ink-acceptingand ink-adhesive (ink-repelling) areas and during driographic printing,only ink is supplied to the master.

Lithographic printing masters are generally obtained by the image-wiseexposure and processing of a radiation sensitive layer on a lithographicsupport or by image-wise jetting of ink onto a lithographic support.Imaging and processing renders the so-called lithographic printing plateprecursor into a printing plate or master. Imaging-wise exposure of theradiation sensitive coating to heat or light, typically by means of adigitally modulated exposure device such as a laser, triggers a(physico-)chemical process, such as ablation, polymerization,insolubilization by cross-linking of a polymer or by particlecoagulation of a thermoplastic polymer latex, solubilization by thedestruction of intermolecular interactions or by increasing thepenetrability of a development barrier layer. Although some plateprecursors are capable of producing a lithographic image immediatelyafter exposure, the most popular lithographic plate precursors requirewet processing since the exposure produces a difference of solubility orof rate of dissolution in a developer between the exposed and thenon-exposed areas of the coating. In positive working lithographic plateprecursors, the exposed areas of the coating dissolve in the developerwhile the non-exposed areas remain resistant to the developer. Innegative working lithographic plate precursors, the non-exposed areas ofthe coating dissolve in the developer while the exposed areas remainresistant to the developer. Most lithographic plate precursors contain ahydrophobic coating on a hydrophilic support, so that the areas whichremain resistant to the developer define the ink-accepting, printingareas of the plate while the hydrophilic support is revealed by thedissolution of the coating in the developer at the non-printing areas.Generation of a lithographic printing master by image-wise jetting ofink on a lithographic support does not require a wet processing duringwhich coating has to be dissolved in a developer. The ink receivinglayer, which acts as the image recording layer, can be the surface of ananodised aluminium lithographic support optionally with a post-anodictreatment or coated with a hydrophilic layer. The jetted ink hasink-accepting properties on the press and reveals the printing areas.

In a typical industrial plate making process, the lithographic coatingis applied on a support which is provided to the coating head as a web.After coating and drying, the web is sliced (cutting in the webdirection) and cut (across the web) to produce individual printing plateprecursors. The plates are then stacked, packed and shipped. Typically,the slicing and cutting produces upward burr, i.e. the support at theplate edges is protruding in the direction of the image recording layer,as shown in FIG. 1. In FIG. 1, the image recording layer is representedby the thin line on top of the cross-section of the lithographicprinting plate precursor. During shipping, the burr often createsscratches on the surface of both sides of the plate, due to slightshifting of the plates in the stack. Another problem caused by prolongedstacking of plates is blocking, i.e. sticking of adjacent plates.Scratches or scuffs can also occur when plates are removed from thestack, either produced by burr or by the separation of sticking plates.Scratches in the image recording layer at the front side of the plateoften produce visible defects in the image area.

In order to reduce the sticking of stacked plates and to avoid scratchesduring transport and/or plate removal, plate manufacturers havetraditionally inserted paper sheets as interleaf between the stackedplates. Unfortunately, however, there are many disadvantages associatedwith the use of these interleaf sheets. In particular, digital plateexposure systems (commonly referred to as platesetters) are increasinglyequipped with automatic plate-feeding devices and consequently, thepresence of interleaf sheets creates serious complications. Such devicesare designed to mechanically remove a plate from a stack and load itonto the exposure device, while additional machinery is required inorder to mechanically remove the interleaf sheets. Moreover, theseinterleaf sheets represent an additional cost, incl. for their disposal.Therefore there is an increasing desire for the elimination of interleafsheets in stacks of lithographic printing plate precursors.

The printing plate precursor described in JP 02/040657 A manages to beproduced without the use of interleaf sheets. A UV-cured layer producedfrom a photopolymerisable material is located on the back side of thealuminium support. In EP 1834802 A an infrared sensitive lithographicprinting plate precursor comprises on one side of a support an imagerecording layer and on the other side a back coat layer having a Vickershardness of 0.2 or less. DE 19908529 A describes a printing plateprecursor comprising a pigmented light-sensitive layer on one side ofits support and a layer comprising an organic polymeric material havinga glass transition temperature of 45° C. or above on the other side.

A lithographic plate precursor which can be stacked without the use ofinterleaf sheets is also described in EP 528395 A. It comprises asupport made of aluminium, a layer of an organic polymeric materialhaving a glass transition temperature of not less than 20° C. with athickness of from 0.01 to 8.0 μm on the back side of the support and alight sensitive layer on the front side of the support. A discontinuousmatting layer consisting of particles is provided on top of the lightsensitive layer. The function of the matting agents is to reduce thesticking of stacked plates by allowing air inclusion between the plates.However, matting layers easily damage the image recording layer uponrelative movement of stacked plates, because the matting agents aretypically inorganic pigment particles of a high hardness. In addition,matting layers, in particular those comprising a particulate materialhaving a low glass transition temperature, tend to stick to the back ofthe overlying plate in the stack. Besides the interaction between thematting agent and the image recording layer, additional technicalproblems occur in the production of pigmented back coatings. One of thedisadvantages is that the solid constituents sediment very quickly,meaning that additional measures are required in order to be able tocoat a homogeneous back layer.

EP1019254A discloses a lithographic printing plate precursor wherein theback side of the support is coated with an inert coating comprising adiscontinuous layer obtained by coating a dispersion or spraying apowder. In EP1217447A a precursor material for the production of offsetprinting plates is disclosed with a continuous pigment free layer on theback side, essentially consisting of an organic polymeric materialhaving a glass transition temperature of at least 45° C. and having asmoothness of from 5 to 800 s. In the same document, applying the backcoat via spraying is disclosed. A disadvantage of spraying methods isthe low yield since up to 50% of the atomised coating solution may bewasted into the environment. Furthermore there is a high risk ofcontaminating the manufacturing equipment.

In EP1239328A a precursor material for the production of offset printingplates is disclosed with a continuous electrically conductive backcoating. The back layer is coated with a hot-melt ink comprising UVcurable organic polymer with a gravure printing technique by means ofengraved rollers. The surface of the back layer is smooth or structured.

EP1156370A describes a production method of a back coating of an offsetplate which is light sensitive. The back coating is not subjected toimage wise exposure. The invention makes it possible to cut a stack ofprinting plate precursors without considerable cutting burrs and damageto the knife.

Further, as disclosed in JP 62-19315A, burr is generated at the surfaceside of the radiation sensitive layer of a plate precursor duringcutting of the precursor web. Several attempts have been made to reduceor prevent this burr in the manufacturing process, such as trimmingcorners of the edge portions of the support member with a file or aknife. However, printing plate precursors have to be taken out one byone to be trimmed. Therefore, this method is not appropriate for use inindustrial manufacturing.

In the printing plate precursor manufacturing process, printing plateprecursors often have to be re-cut after they have been stacked.Re-cutting of plate stacks is usually done with small size adaptations(typically 5 mm or less) of the plate size or plates have to be re-sizedfrom for example an 8-up size to a typical 4-up size.

None of the prior art documents which disclose the use of back coatingsin order to avoid the use of interleafs, provides a solution for theproblem that, in a stack of plates, the upward burr (i.e. burrprotruding towards the image recording side of the plate) which isproduced by the slicing and cutting of the plates on the manufacturingline, interacts with the coating on the back side of the adjacent plateduring shipping or prolonged storage. Upward burr even occurs when thecutting is carried out from the image recording side of the plate. In astack of plates, the burr may anchor into the back coating of theadjacent plate increasing stickiness between the plates, which oftenleads to plate separation problems in the automatic plate loader of theplatesetter.

Moreover, none of the prior art documents provides a solution for theproblem that, when a stack of plates without interleaf has to be re-cut,the very strong contact between the burr produced by the high pressureduring the re-cutting and the back coating of the adjacent plate resultsin the sticking of the plates to each other at the edges. It istherefore highly desirable to provide a solution to avoid sticking oflithographic printing plate precursors after re-cut of a stack of platesand still providing, without the need for interleaf sheets, sufficientprotection of the image recording layer against mechanical damage.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a solution whichenables stacking and recutting of lithographic printing plate precursorswithout the need for interleafs and which reduces the risk that adjacentplates tend to stick to each other. A method for manufacturing alithographic printing plate precursor is defined below. Compared to theprior art using back coatings which extend over the full area of theplate, sticking of adjacent printing plate precursors produced accordingto preferred embodiments of the present invention is significantlyreduced without impairing the protection of the image recording layeragainst scratching. Also the risk of burr anchoring into the back layer,obtained according to preferred embodiments of the present invention, issignificantly reduced by the lower weight and/or surface coverage of theback layer at the edges of the plate.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention. Specificembodiments of the invention are also defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of the edge of a lithographic printing plateprecursor showing the burr protruding from the front side of thesupport, created by cutting.

FIG. 2 is a preferred embodiment of a non-continuous back layer of alithographic printing plate precursor, distributed as dots according toa regular pattern.

FIG. 3 is a preferred embodiment of a non-continuous back layer of alithographic printing plate precursor, randomly distributed as dots.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for manufacturing lithographic printing plate precursorsaccording to a preferred embodiment of the present invention comprisesthree essential steps: (a) providing a support as a web, coating animage recording layer on the front side of the support, and depositing aback layer on the back side of the support.

The Back Layer Method of Making the Back Layer

A method of making the back layer according to a preferred embodiment ofthe present invention comprises the deposition of a liquid on the backside of the support, by a deposition technique which is capable ofdepositing a back layer according to a predefined image. The liquid maysubsequently be solidified by phase change, curing and/or drying(regardless of the mechanism of solidification, all said liquids shallbe referred to hereafter as “curable liquids”). In industrial platemanufacturing methods, the image recording layer(s) and the backlayer(s) are typically applied (in any order) on a web as support andsubsequently dried and cured.

In a preferred embodiment, the curable liquid is applied as anon-continuous patterned layer, e.g. as distinct dots and/or lines, orby leaving voids in the back layer (a void is to be understood as anarea which does not contain any material or which contains less materialthan the surrounding areas). As a result, the non-continuous layer ispreferably characterized by an average surface coverage. Surfacecoverage shall be understood as the percentage of the surface area whichis covered by the back layer. Preferably, the surface coverage of theback layer has a value of 50% or less. More preferably, the dot surfacecoverage is 10% or less, most preferably between 1% and 5%. The averagedistance between the dots is preferably between 0.5 and 10.0 mm, morepreferably between 1.0 and 5.0 mm, most preferably between 1.0 and 2.0mm. The surface coverage of the back layer can be constant over the webor can have locally different values. Hence, a preferred embodiment ofthe invention also reduces the dry coating weight of the back layer to aminimum without impairing the protection against scratching the imagerecording layer, leading to an increased production speed or reducedcost of production.

The dots, lines and/or voids of the non-continuous back layer may bearranged according to a regular pattern, comparable with an amplitudemodulated half tone screen, based on a geometric and fixed spacing ofdots. The spacing and dot size of these dots, lines and/or voids mayvary depending on the required average surface coverage of the backlayer (for example, dots or voids having a diameter or lines having awidth from 10 to 200 micrometres). A preferred embodiment of thedistribution of the dots is illustrated in FIG. 2. which shows the fouredges 1 of a printing plate precursor and a back layer consisting of aregular pattern of dots 2. Preferably, the surface coverage of the backlayer as dots in a regular pattern has a value of 50% or less. Morepreferably, the dot surface coverage is 10% or less, most preferablybetween 1% and 5%. The average distance between the dots is preferablybetween 0.5 and 10.0 mm, more preferably between 1.0 and 5.0 mm, mostpreferably between 1.0 and 2.0 mm. A regular pattern of dots can becharacterised by two perpendicular axes wherein at least one axis isformed by connecting the dots with a straight line in the directioncorresponding with a minimal distance between the dots.

It is preferable that lines of the pattern or the dots of the regularpattern do not coincide with one or more predefined edges. Indeed, thiswould increase the risk that the burr originating from the cutting wouldanchor in the back coating as dots or lines present at the edges. Inorder to avoid that the lines of the pattern coincide with the edges ofthe plate, the lines can be deposited in such a way that they show anangle θ greater than 0° and equal or smaller than 45° with respect tothe moving direction of the web. In order to avoid that the dots of aregular pattern coincide with the edges of the plate, the dots can bedeposited in such a way that the axis of the pattern which orientationis the closest with the moving direction of the web show an angle θgreater than 0° and equal or smaller than 45° with respect to the movingdirection of the web. Preferably, the angle θ has a value of 15°.

In another embodiment the dots may be arranged according to stochasticscreening or frequency-modulated screening using a fixed size of dots(for example, a diameter of about 25 micrometres). The distributiondensity of the dots may vary depending on the required average surfacecoverage of the back layer. A preferred embodiment of the distributionof the dots is illustrated in FIG. 3, showing the edge 1 of a printingplate precursor and the dot pattern of the back layer randomly arranged2.

In a method of the invention, the pattern of the deposited back layersas described above is obtained by a predefined image. The predefinedimage contains the placement, spacing and size of the dots, voids and/orlines. The predefined image is preferably a binary image, i.e. an imagewhich consists of pixels. The binary image may be obtained by anysuitable means, e.g. it may be recovered from any image file formatincluding vector file formats.

In a preferred embodiment of the invention, the predefined image maycontain also the cutting scheme. The cutting scheme is a scheme whichdefines the edges, the so-called predefined edges, of the individualprinting plate precursor sheets to be obtained after slicing andcutting. The scheme is typically based on the required sizes of theprinting plate precursor sheets, the position in the web, and therequired number of printing plates from a manufacturing batch. Based onthe cutting scheme, the deposition of the back layer can be done in sucha way that the back layer has an average weight and/or surface coverageat the edges of the plate sheet which is lower than the average weightand/or surface coverage of the total back layer, i.e. the average weightof the back layer over the full surface of the plate sheet aftercutting. More particularly, according a method of the invention, thedeposition of the back layer in production can be done in such a waythat the average weight and/or surface coverage of the back layer in thesections within 1 cm from the predefined edges (hereafter also referredto as the “edge section”) is lower than in the neighbouring sections inthe web. The weight of the back layer is to be understood as the dryweight of the solid layer, i.e. after drying and curing of the appliedliquid.

In an embodiment of the present invention, a lower average weight in theedge section(s) can be obtained by applying the curable liquid with adeposition method as described hereafter, thereby applying lessmaterial, or no material at all, near the predefined edges and obtaininga lower average layer thickness in the edge section, compared to theplate sheet as a whole.

In a preferred embodiment of the invention, the non-continuous backlayer can be combined with one ore more continuous underlying layersbetween said back layer and the support. This can be obtained by firstapplying one or more continuous layers via one of the deposition methodsas described below or by a conventional coating method as gravure, nip,metered roller methods, dip coating or slot coating. Subsequently thenon-continuous back layer can be applied via one of the methodsdescribed below. In the embodiments comprising multiple layers at theback side of the plate, the features “average weight” shall relate tothe complete back coating, i.e. the combination of all the layersprovided on the back side of the support of the plate.

In order to provide sufficient protection to the image recording layerof the plate in contact with said back layer, the average thickness ofthe back layer and the optional underlying layer(s) is preferably morethan 10 μm, more preferably more than 30 μm, most preferably between 40and 60 μm. The average thickness of the back layer is defined as theaverage thickness of the back layer, measured between the top of thelayer, being the combination of all the layers provided on the back sideof the support of the plate, over the whole plate precursor and thebottom of the layer which contacts the support. To prevent mechanicaldamage of the image recording layer, the hardness of the layerpreferably has a Shore A hardness of 90 or less, more preferably 85 orless. Another preferred embodiment of the invention providescompositions which lead after curing to back layers which are soft andflexible to avoid any damage of the image recording layer in contactwith the back side of the plate. It is an advantage of the presentinvention that with the back layer, the friction coefficient of theplate precursor sheets in contact with each other can be controlled tooptimise transport in the platesetter and minimise damaging of the imagerecording layer containing plate precursor sheets during transport.

In a preferred embodiment, the deposition of the curable liquids isdirectly obtained from the predefined image using a printing techniquefrom the group of ink jet printing, valve jet printing and aerosoljetting.

In another preferred embodiment, a printing master is first madeaccording to the predefined image. This printing master is then used todeposit the curable liquid according to gravure or screen printingtechniques.

Preferred embodiments of the invention are particularly useful whenstacked printing plate precursor sheets have to be re-cut. Sincere-cutting usually is done with small size adaptations (typically 5 mmor less) of the plate size, the lower average weight of the back layerin the edge section(s) prevent sticking of stacked plates at the edgesafter re-cut. Re-cutting can also be necessary when for example platesheets have to be re-sized from an 8-up size to a typical 4-up size. Asa result, it is preferred to include additional sections having a loweraverage weight and/or surface coverage in the cutting scheme which arenot located at the edges of the sheet that is obtained after applyingthe cutting scheme, but which correspond to the edges of smallerformats; as a result, re-cutting of stacked plate sheets into saidsmaller formats is possible without risk of sticking of adjacentprinting plate precursor sheets.

After slicing and cutting, the lithographic printing plate precursorsare stacked on top of each other without interleaf sheets. From thesestacks packs of between 10 and 100 plates, more preferably between 30and 50 plates are made and put in a box or sleeve, preferably acardboard box or sleeve so as to obtain boxes of lithographic printingplates without interleaf sheets, wherein the printing plate precursorshave a back layer produced according to a method of the presentinvention.

Deposition Techniques

Preferred deposition techniques according to the invention are3D-printing, ink jet printing, aerosol jet printing, valve-jet printing,gravure printing and screen printing.

A. Ink Jet Printing

Ink jet printing is one of the more preferable deposition techniques tobe used in the invention. The inkjet printer includes any dischargingdevice capable of breaking up a liquid into small drops which are thendirected onto the surface of the support. In the most preferredembodiment the radiation curable liquids are jetted by one or moreprinting heads ejecting small droplets in a controlled manner throughnozzles onto the support which is moving relative to the printinghead(s). A preferred printing head for the inkjet printing system is apiezoelectric head. Piezoelectric inkjet printing is based on themovement of a piezoelectric ceramic transducer when a voltage is appliedthereto. The application of a voltage changes the shape of thepiezoelectric ceramic transducer in the printing head creating a void,which is then filled with radiation curable liquid. When the voltage isagain removed, the ceramic returns to its original shape, ejecting adrop of liquid from the print head. However the inkjet printing methodis not restricted to piezoelectric inkjet printing. Other inkjetprinting heads can be used and include various types, such as acontinuous type and thermal, electrostatic and acoustic drop on demandtypes. An example of a print head according to the current invention iscapable to eject droplets having a volume between 0.1 and 100 picoliter(pl) and preferably between 42 and 100 pl. EP 2420382 A, EP 2420383A, EP2465678 A and EP 2371541A disclose preferred constellations of multipleprint heads, preferably back to back print heads.

B. Aerosol Jet Printing

Another preferable deposition technique according to a method of theinvention is Aerosol Jet Printing. This technique which has beendeveloped by Optomec, preserves most of the advantages of inkjetprinting, while reducing many of its limitations. The technique isdisclosed in for example US 20030048314, US 20030020768, US 20030228124and WO 2009/049072. An Aerosol Jet Print Engine is commerciallyavailable from Optomec, for example the Aerosol Jet Printer OPTOMEC AJ300 CE. More details on the Aerosol Jet Printing technique and engineare found on the Optomec website www.optomec.com. Virtually any liquidhaving a viscosity less than 5000 mPa·s can be deposited using theAerosol Jet Printing technique while inkjet printing requires liquidshaving a viscosity of less than 20 mPa·s. In an Aerosol Jet Printingsystem, rather than producing individual drops of ink, an aerosol isproduced, focused and directed toward the substrate. The dischargingdevice consists of two key components: a first module for forming anaerosol from a liquid and a second module focussing the aerosol afterdriving out of the nozzle and depositing the aerosol droplets on asupport. Similar to continuous inkjet, this aerosol stream can beshuttered to interrupt the stream.

In Aerosol Jet Printing a collimated beam of material is discharged on asubstrate. This allows the resolution to be maintained over a wide rangeof standoff (head to substrate) distances. This enables larger standoffdistances to be used than are possible with inkjet printing. In AerosolJet Printing, a collimated beam of material is formed which is thendeposited on the support. Therefore, standoff distances from 1 to 5 mmcan be used without loss of resolution. This is important in aproduction environment in which supports of different thickness areused, because no adaptation to the standoff is hence required.

In Aerosol Jet Printing, two different ways of forming an aerosol can beemployed, depending on the characteristics of the material to bedeposited. An ultrasonic transducer can be used for nebulising lowviscosity liquids (0.7 to 10 mPa·s). Here, a piezoelectric transducerproduces high frequency pressure waves, which are transmitted through acoupling liquid (typically water) into the deposition liquid. For higherviscosity liquids (10 to 5000 mPa·s) or larger suspended particles (<0.5μm) pneumatic atomization is used. In this technique, a high velocitygas stream is used to shear the liquid stream into droplets.

C. Valve-Jet Printing

Valve-jetting is a type of drop-on-demand jet technique that typicallyuses a discharging device with a solenoid to open and shut a valve andwhich is a suitable deposition technique for the invention. Valve-jettechnology is described in US 2010/0132612 A. The liquid behind thevalve is under pressure and as the valve is opened, a drop of the liquidis shot from the valve and travels through the nozzle to the support.The drop frequency is typically in the range from 2 to 4 kHz, althoughnot limited tot this range. In a preferred embodiment, pressurized gasis used for driving the liquid out of the valve-jet print head.Valve-jet is a suitable deposition method to produce back layersaccording to the invention. The viscosity of the liquid for valve-jetprinting may be higher than for ink jet printing. This leads to abroader range of useable compounds in the liquid such as oligomers andpolymers compared to liquids having low viscosity requirements. Theviscosity of the liquid to be jetted by valve-jet according to apreferred embodiment of the invention is preferably higher than 50 cPs,more preferably higher than 100 cPs. Valve-jet is also a suitabletechnique because it generates droplets with a volume in a range of 1 to50 nl, preferably 5 to 15 nl. These drop volumes give rise to a backlayer wherein the required thickness of the back layer is achieved bydepositing only one layer of liquid. The less layers have to bedeposited to achieve the required thickness of the back layer, thehigher the throughput is of the back layer deposition process. Valve-jetprint heads are known in the art and commercially available, e.g. fromMatthews Swedot AB in Sweden.

D. Gravure Printing

In gravure printing technology, cells are image wise engraved into thesurface of a cylinder forming a printing master. The liquid istransferred from the engraved cells to the support by a high printingpressure. With this technique, back layers with a thickness 5 to 8 μmcan be obtained in one pass, meaning after one revolution of thecylinder. If higher thicknesses of the back layer are required, multiplecylinders can be arranged one after each other.

E. Screen Printing

Screen printing is a printing technique that uses a woven mesh tosupport a liquid-blocking stencil to receive a desired image. Since theliquid is forced through the screen, the screen printing stencildetermines according to which pattern, liquid is transferred to thesupport. The woven mesh together with the screen printing stencil is theprinting master. Following screen printing methods such as flat-to-flatmethod, flat-to-round method and rotary printing can be used. In aproduction environment, using a continuous web support, rotary screenprinting is the most preferred one. Screen printing is particularlypreferred due to the high layer thickness of the back layer that canthus been obtained in one single pass.

Configuration of the Discharging Devices

In a preferred embodiment of the discharging devices, the nozzles of theprint heads are arranged at least over the entire width of the web. Toobtain the required thickness of the back layer, the nozzle structure ofthe discharging devices can be constructed as to allow for a pluralityof rows can be arranged in succession, the number depending on therequired thickness of the back layer and the drop volume discharged bythe discharging device. To build the back layer, and more specificallythe non-continuous layer sufficiently efficient, the radiation source ispreferably mounted next to and behind the print head. In thisembodiment, the jetting heads are fixed and do not have to move back andforth over the width of the lithographic substrate.

A combination of a shuttle which carries a plurality of print heads isalso a possible implementation for an arrangement to implement theinvention. The shuttle moves back and forth over the width of thesupport. A shuttle holds the print head constellation in headpositioning devices and liquids supplies for the liquid. The shuttlearranges the positioning of the head positioning devices to correct foreach print head the distance between the print head and the surface ofthe support. A shuttle frame connects the shuttle to the base frame ofthe printing device. It supports accuracy less than 15 μm, preferablyless than 8 μm in all positions from the shuttle to the support bycomprising preferably a high resolution encoder system and preferably alinear magnetic motor. The shuttle can be moved away from the web to amaintenance purge position to inspect and service the shuttle.

A liquid supply delivers the liquid to the discharging devices inoptimized conditions for jetting. The liquid supply comprises preferablya degassing unit to filter the liquid and degas the liquid below 40% andpreferably a manifold wherein a static pressure is adjusted so thenozzle column in a print head is under optimal conditions which dependson the level in the manifold and the nozzle plate of the print head. Theliquid supply comprises preferably a valve to prevent a print head fromleaking or sucking air into the nozzles of the print head. In case of ashuttle which carries the discharging devices, the liquid supply is partof the shuttle. In one embodiment of the invention, the curing device ismaking part of the shuttle, assuring a short time between printing andcuring.

In a preferred embodiment, 3-D printing is used to obtain anon-continuous back layer. The 3D-printing method comprises providingthe predefined image consisting of a plurality of image pixels andgenerating an image by applying a topographic operator to the predefinedimage to generate for every image pixel a representation of a pixelheight profile. Thereby a plurality of image layers from the filteredimage is generated. The curable liquids are applied by one of thedeposition techniques described above via subsequent layerscorresponding with the image layers generated by the topographicoperator and whereby an applied layer is preferably immobilized using acuring device before a subsequent layer is applied. The curing does nothave to be a full cure, but can be a partial cure. Optionally some ofthe layers are not cured directly after deposition of a layer, but afterdeposition of a subsequent layer. In a preferred embodiment, eachapplied layer is immobilized using the curing device before a subsequentlayer is applied. Preferably, any cross-section through a solid sectionof a 3-D structure at a second level in the 3-D print structure which iscloser to the substrate than a first level has an area which is equal toor larger than the area of the cross-section of the 3-D structure at thefirst level.

Curing

Suitable methods for solidifying deposited curable liquids includethermal curing by heat or IR light and exposure to actinic radiation(e.g. UV light or electron beam radiation).

The curing of the deposited back layer can preferably be done,immediately after the deposition of the liquid by exposing the liquid toa curing source. This provides immobilization often referred to as“pinning” and may be very useful for curing droplets to build completelayers. After this pinning, a full curing can be performed.

The curing step included in a method of the invention can be “partial”or “full”. The terms “partial curing” and “full curing” refer to thedegree of curing, i.e. the percentage of converted functional groups,and may be determined by, for example, RT-FTIR (Real-Time FourierTransform Infra-Red Spectroscopy) which is a method well known to theone skilled in the art of curable formulations. Partial curing isdefined as a degree of curing wherein at least 5%, preferably 10%, ofthe functional groups in the deposited liquid is converted. Full curingis defined as a degree of curing wherein the increase in the percentageof converted functional groups with increased exposure to heat orradiation (time and/or dose) is negligible. Full curing corresponds witha conversion percentage that is within 10%, preferably 5%, from themaximum conversion percentage.

Preferably the curing process to be used in the invention is performedby UV radiation.

The curing device may be arranged in combination with the printingdevice, travelling therewith so that the curable liquid is exposed tocuring radiation very shortly after been jetted. It may be difficult toprovide a small enough radiation source connected to and travelling withthe printing device. Therefore, a static fixed radiation source may beemployed, e.g. a source of UV-light, which is then connected to theprinting device by a flexible radiation conductor such as a fibre opticbundle or an internally reflective flexible tube.

Alternatively, a source of radiation arranged not to move with theprinting device, may be an elongated radiation source extendingtransversely across the web based support.

Any UV light source, as long as part of the emitted light can beabsorbed by the photo-initiator or photo-initiator system of the liquiddroplets, may be employed as a radiation source, such as, a high or lowpressure mercury lamp, a cold cathode tube, a black light, anultraviolet light emitting diode (LED), an ultraviolet laser, and aflash light. LEDs are preferred because they allow a more compact designof the curing apparatus.

The most important parameters when selecting a curing source are thespectrum and the intensity of the UV-light. Both parameters affect thespeed of the curing. Short wavelength UV radiation, such as UV-Cradiation, has poor penetration capabilities and enables to curedroplets primarily on the outside. A typical UV-C light source is a lowpressure mercury vapour electrical discharge bulb. Such a source has asmall spectral distribution of energy, with only a strong peak in theshort wavelength region of the UV spectrum. Long wavelength UVradiation, such as UV-A radiation, has better penetration properties. Atypical UV-A source is a medium or high pressure mercury vapourelectrical discharge bulb. Recently UV-LEDs have become commerciallyavailable which also emit in the UV-A spectrum and that have thepotential to replace gas discharge bulb UV sources. By doping themercury gas in the discharge bulb with iron or gallium, an emission canbe obtained that covers both the UV-A and UV-C spectrum. The intensityof a curing source has a direct effect on curing speed. A high intensityresults in higher curing speeds.

The curing speed should be sufficiently high to avoid oxygen inhibitionof free radicals that propagate during curing in anaddition-polymerisation process. Such inhibition not only decreasescuring speed, but also negatively affects the conversion ratio ofmonomer into polymer. To minimize such oxygen inhibition, the imagingapparatus preferably includes one or more oxygen depletion units. Theoxygen depletion units place a blanket of nitrogen or other relativelyinert gas (e.g. CO₂), with adjustable position and adjustable inert gasconcentration, in order to reduce the oxygen concentration in the curingenvironment. Residual oxygen levels are usually maintained as low as 200ppm, but are generally in the range of 200 ppm to 1200 ppm.

Another way to prevent oxygen inhibition can be achieved by a lowintensity pre-exposure before the actual curing, thereby obtaining apartially cured liquid phase which is solidified but still containsresidual monomer. This approach improves the adhesion properties betweenthe layers that are subsequently deposited on top of each other. It ispreferred to perform the partial curing with UV-A radiation.

A final post curing however is often realized with UV-C light or withbroad spectrum UV light. Final curing with UV-C light has the propertythat the outside skin of the back layer is fully hardened and do notshow any stickiness.

In one of the preferred embodiments, a curing station includes an UV LEDbar. In the case the printing heads are configured in a shuttle, thecuring station is preferably also mounted in a shuttle which follows themovement of the head(s). To prevent UV light from reaching the nozzleplates preferably anti-scattering profiles are installed, preferablytangential to the support. The UV LED bar comprises 1 or more UV LEDmodules which comprises one or more LED tiles which can be controlledseparately. The anti-scattering profiles preferably comprise to spray athin layer of nitrogen gas or an inert gas to decrease oxygen inhibitionand to improve the curing process. To prevent the warming up of thesupport an air-knife may be added to the curing station that sprayscompressed air directly onto the surface of the support.

The curing can also be performed by an electron beam (EB) curing stationcomprising an electron beam curing system that operates at a voltagethat is adjustable from 125 to 150 kV. The electron beam curing systempreferably provides a cure dose of 0.5 to 6 megarads (Mrads) whenoperating at a process speed between up to 216 m/min. EB curing may bevery well suited to be used for curing in a production environment ofprinting plate precursors since no UV radiation is present which couldirradiate the radiation sensitive layer of the printing plate precursor.

Composition of the Curable Liquids

Any of the ingredients described in sections A-E below are examples ofmaterials which are suitable for making the curable liquid. The curableliquids may be prepared as known in the art by mixing or dispersing theingredients together, optionally followed by milling, as described forexample in EP 1637322 A paragraph [0108] and [0109].

A. Liquids Solidified by Drying

Suitable liquids contain polyolefins (such as polyethylene,polypropylene, polybutylene, polubutadiene of polyisorene), polyesters,polycarbonates, polyamides, polysiloxanes, polystyrene, homopolymers ofcopolymers of or with alkyl acrylate or alkyl methacrylate units (suchas polymethyl methacrylate (PMMA) or styrene-methyl methacrylatecopolymers), polyvinyl acetal, phenoxy resins (for example resins madefrom bisphenol A and epichlorohydrin), polyvinyl chloride (PVC) orpolyvinylidene chloride (PVDC). Other polymers which can be used inliquids giving a microstructuration upon drying are thermoplasticelastomers. Typical thermoplastic elastomers are block- or graftcopolymers having soft and hard segments which phase segregate, forexample Kraton grades available from Kraton Performance Polymers Inc.and Nanostrength grades available from Arkema. The Kraton D SBS blockcopolymers are composed of blocks of styrene and butadiene. Kraton Gpolymers are second generation styrene block copolymers with ahydrogenated midblock of styrene-ethylene/butylene-styrene (SEBS) orstyrene-ethylene/propylene-styrene (SEPS). Kraton FG polymers are SEBSpolymers with maleic anhydride (MA) grafted onto the rubber midblock.Nanostrength® is a new family of self-assembling block copolymerscomprising three linear chain blocks covalently bonded to one another.ARKEMA's commercial range of Nanostrength® “MAM” is based on symmetricacrylic triblocks containing a poly (butyl acrylate) centre block andend blocks of poly (methyl methacrylate). Another class of polymerswhich can be structured during physical drying are ionomers. Suitableexamples for this invention are polymers with the trade name Surlyn andpolymers disclosed in LANTMAN, C. W, et al. Structural Properties ofIonomers. Annual Review of Materials Science. August 1989, vol. 19, p.295-317.

In another preferred embodiment of the invention, the liquid includepolymer latex particles dispersed in a liquid carrier. Latex containingliquid can be heat cured to soften and flow the polymer latex particles.Latex is a liquid suspension comprising a liquid (such as water and/orother liquids) and polymeric particulates from 20 nm to 500 nm (andoften from 100 nm to 300 nm) in size. Typically, the polymericparticulate can be present in the liquid from 0.5 wt % to 20 wt %. Suchpolymeric particulates can comprise a plurality of monomers that aretypically randomly polymerized, and can also be cross linked.Additionally, in one implementation, the latex component can have aglass transition temperature from about −20 to +100 degrees C. Forexample, the size of the latex particles size can range fromapproximately 100 to 350 nanometres. For example, latex may be preparedby using emulsion polymerization of various ratios of monomer such, butare in no way limited to, methyl methacrylate, styrene, various ‘soft’acrylate esters, and functionalized monomers. These functionalizedmonomers include ‘vinyl’ monomers containing hydroxyl groups, carboxylicacids, sulfonic or sulphate acids and phosphate acids, where ‘vinyl’denotes derivatives of acrylates, methacrylates, functionalized styrene,allyl ether and esters, vinyl ethers as selected examples. Monomers thatare often used include ethyl acrylate; ethyl methacrylate; benzylacrylate; benzyl methacrylate; propyl acrylate; propyl methacrylate;iso-propyl acrylate; iso-propyl methacrylate; butyl acrylate; butylmethacrylate; hexyl acrylate; hexyl methacrylate; octadecylmethacrylate; octadecyl acrylate; lauryl methacrylate; lauryl acrylate;hydroxyethyl acrylate; hydroxyethyl methacrylate; hydroxyhexyl acrylate;hydroxyhexyl methacrylate; hydroxyoctadecyl acrylate; hydroxyoctadecylmethacrylate; hydroxylauryl methacrylate; hydroxylauryl acrylate;phenethyl acrylate; phenethyl methacrylate; 6-phenylhexyl acrylate;6-phenylhexyl methacrylate; phenyllauryl acrylate; phenyllaurylmethacrylate; 3-nitrophenyl-6-hexyl methacrylate;3-nitrophenyl-18-octadecyl acrylate; ethyleneglycol dicyclopentyl etheracrylate; vinyl ethyl ketone; vinyl propyl ketone; vinyl hexyl ketone;vinyl octyl ketone; vinyl butyl ketone; cyclohexyl acrylate;methoxysilane; acryloxypropyhiethyldimethoxysilane; trifluoromethylstyrene; trifluoromethyl acrylate; trifluoromethyl methacrylate;tetrafluoropropyl acrylate; tetrafluoropropyl methacrylate;heptafluorobutyl methacrylate; iso-butyl acrylate; iso-butylmethacrylate; 2-ethylhexyl acrylate; 2-ethylhexyl methacrylate;iso-octyl acrylate; and iso-octyl methacrylate. The latexes used hereincan be prepared by latex emulsion polymerization, and, in oneimplementation, can have a weight average molecular weight from 10,000Mw to 5,000,000 Mw. This range is only illustrative and can be broader.Co-polymers can be formed, including block copolymers, randomlyassembled copolymers, copolymers including crosslinkers, or the like.Often the copolymer is a randomly assembled copolymer, though varioussubclasses of each polymer type can be used, e.g., core-shell, variousglass transition temperatures, surface acid groups, cross linking, etc.Also mixtures of different latexes can be used having complementaryreactivity e.g. combination of a epoxy functional latex with a amino- orhydroxy functional latex forming a cross linked network upon heating dueto covalent reaction or combination of an anionic and cationic latexforming a cross linked network due to ionic interaction or latex withmaleimide combined with another latex with vinyltriazine forming a crosslinked network due to hydrogen interaction.

B. Heat Curable Liquid

Heat curable liquids typically comprise thermoset polymers. Preferredexamples of thermoset polymers are polyurethanes, novolak resins, ureaformaldehyde resins, cyanate esters of polycyanurates and epoxyresins.

Other preferable polymers which can be comprised in heat curable liquidsare polymers which can irreversibly cure. The curing may be done througha chemical reaction, a crosslinking reaction, which can be acceleratedby heat or through a reaction initiated by a thermal initiator. Thecross linking reaction occurs between crosslinkable (co-) polymers orbetween polymers and a cross linking agent. Crosslinkable (co-) polymersare (co-) polymers which contain a functional group (A group) and afunctional group (B group) wherein both groups can react with each otherand lead to a cross linking reaction upon heating. The groups A and Bcan be both present in the same polymer (self-cross linkable copolymers)or in a mixture of a polymers, each polymer containing a group A or B.Preferable functional groups A and B are: carbodiimide and carboxylicacid; anhydride and alcohol; anhydride and amine; isocyanate andalcohol; isocyanurate and alcohol; isocyanate and amine; epoxy andamine; epoxy and alcohol, ketone and dihydrazide. Besides the reactionbetween reactive polymers, curing can also take place by reaction of apolymer having a functional group such as A, B, ketone, Si-vinyl and across linking agent. Suitable cross linking agents are: silanes,dihydrazide, carbodiimide, isocyanates, isocyanurates, amines, alcohols,carboxylic acids, anhydrides. Curing by heat can also be obtained byusing thermal initiators in combination with polymers having unsaturatedgroups (allyl, . . . ) or unsaturated hydrocarbons. Thermal initiator(s)suitable for use in the curable composition include tert-amylperoxybenzoate, 4,4-azobis(4-cyanovaleric acid),1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azobisisobutyronitrile(AIBN), benzoyl peroxide, 2,2-bis(tert-butylperoxy)butane,1,1-bis(tert-butylperoxy)cyclohexane, 1,1-Bis(tert-butylperoxy)cyclohexane,2,5-bis(tert-butylperoxy)-2,5-dimethylhexane,2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne,bis(1-(tert-butylperoxy)-1-methylethyl)benzene,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylhydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butylperoxy benzoate, tert-butylperoxy isopropyl carbonate, cumene hydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroyl peroxide,2,4-pentanedione peroxide, peracetic acid and potassium persulfate.

The cross linking can occur through covalent bonding/reaction but alsodue to hydrogen bonding or ionic interactions. Additional examples aredisclosed in LABANA, S. Encyclopedia of polymer sciences andengineering. 2nd edition. New York: Wiley John, 1986. p. Vol 4, 350-390,LLOYD, William G. Thermal initiation and branching. Chemical Technology.March 1971, p. 176-180. and BEVINGTON, John C. Initiation ofpolymerization: azo compounds and peroxides. Makromolekulare Chemie,Macromolecular Symposia. 1987, vol. 10-11, p. 89-107.

In another preferable embodiment, thermal initiators can be combinedwith addition-polymerisable acrylates or methacrylates. Usefuladdition-polymerisable acrylates or methacrylates are described in thesection of radiation curable liquids.

C. Radiation Curable Liquids

Radiation curable liquids which are suitable for the invention maycomprise organic polymeric material, monomeric or oligomeric compoundswhich polymerize, condense or crosslink on exposure to radiation.Particularly suitable for this purpose are addition-polymerisableacrylates or methacrylates. These can be monofunctional, difunctional ormultifunctional, such as ethyl (meth) acrylate, propyl (meth) acrylate,butyl (meth) acrylate, trimethylolpropane mono-, di, or tri (meth)acrylate or pentaerythritol tri (meth) acrylate. Also suitable are(meth)acrylamides, such as N-methyl-, N-propyl-, N-butyl- orN-isobutyl-(meth)acrylamide; furthermore allyl esters, such as allylacetate; vinyl ethers, such as butyl vinyl ether, octyl vinyl ether,decyl vinyl ether, 2-methoxyethyl vinyl ether, diethylene glycol vinylether of benzyl vinyl ether; polyfunctional urethane acrylates.Particular suitable compounds for radiation curable liquids are:monofunctional (meth) acrylate monomers, difunctional (meth) acrylatemonomers, multi functional (meth) acrylate monomers and urethaneacrylate oligomers will now be described.

Monofunctional (Meth)Acrylate Monomer

Any monofunctional (meth) acrylate monomer, such as disclosed forexample in EP 1637322 A, paragraph [0055], may be used.

However, the curable liquid preferably comprises a cyclic monofunctional(meth) acrylate monomer. Examples of such cyclic monofunctional(meth)acrylates are isobornyl acrylate (SR506D from Sartomer),tetrahydrofurfuryl methacrylate (SR203 from Sartomer),4-t.butylcyclohexyl acrylate (Laromer TBCH from BASF),dicyclopentadienyl acrylate (Laromer DCPA from BASF), dioxalanefunctional acrylates (CHDOL10 and MEDOL10 from San Esters Corporation),cyclic trimethylolpropane formal acrylate (SR531 from Sartomer),2-phenoxyethyl acrylate (SR339C from Sartomer), 2-phenoxyethylmethacrylate (SR340 from Sartomer), tetrahydrofurfuryl acrylate (SR285from Sartomer), 3,3,5-trimethyl cyclohexyl acrylate (CD420 fromSartomer). Particularly preferred cyclic monofunctional (meth)acrylatesmonomers are isobornyl acrylate (IBOA) and 4-t.butylcyclohexyl acrylate(Laromer TBCH from BASF).

For jetting purposes, the amount of the cyclic monofunctional (meth)acrylate monomer is preferably at least 25 wt %, more preferably atleast 30 wt %, relative to the total weight of the curable liquid.

Difunctional (Meth)Acrylate Monomer

A preferred difunctional (meth) acrylate monomer is a polyalkyleneglycol di (meth) acrylate. Such compounds have two acrylate ormethacrylate groups attached by an ester linkage at the opposite ends ofa hydrophilic polyalkylene glycol. Typically, the longer the length ofthe polyalkylene chain, the softer and more flexible the obtained layerafter curing.

Examples of such polyalkylene glycol di(meth)acrylates include:

1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate,diethylene glycol diacrylate, diethylene glycol dimethacrylate,dipropylene glycol diacrylate, ethylene glycol dimethacrylate,polyethylene glycol (200) diacrylate, polyethylene glycol (400)diacrylate, polyethylene glycol (400) dimethacrylate, polyethyleneglycol (600) diacrylate, polyethylene glycol (600) dimethacrylate,polyethylene glycol dimethacrylate, polypropylene glycol (400)dimethacrylate, tetraethylene glycol diacrylate, tetraethylene glycoldimethacrylate, triethylene glycol diacrylate, triethylene glycoldimethacrylate, tripropylene glycol diacrylate, tripropylene glycoldiacrylate, and combinations thereof. The number between brackets in theabove list refers to the Molecular Weight (MW) of the polyalkylenechain.

Highly preferred polyalkylene glycol diacrylates are polyethylene glycoldiacrylates. Specific examples of commercially available polyethyleneglycol diacrylate monomers include SR259 [polyethylene glycol (200)diacrylate], SR344 [polyethylene glycol (400) diacrylate], SR603[polyethylene glycol (400) dimethacrylate], SR610 [polyethylene glycol(600) diacrylate], SR252 [polyethylene glycol (600) dimethacrylate], allSartomer products; EBECRYL 11 [poly ethylene glycol diacrylate fromCytec; Genomer 1251 [polyethylene glycol 400 diacrylate] from Rahn.Polyethylene glycol (600) diacrylate, available as SR610 from Sartomer,is particularly preferred.

Other preferred difunctional acrylate or methacrylate monomers are e.g.butane diol diacrylate, alkoxylated hexanediol diacrylate, alkoxylatedneopentyl glycol diacrylate and alkoxylated hexanediol dimethacrylate.

The amount of the difunctional (meth) acrylate monomer is preferably atleast 10 wt % of the total monomer content.

Particularly preferred difunctional (meth)acrylate monomers are thoseaccording to Formula I or II,

wherein

k and m in Formula I is an integer ranging from 0 to 5,

l in Formula I is an integer ranging from 1 to 20

n in Formula II is 1, 2, 3 or 4,

R is H or CH₃, and

R′ is H or an alkyl group.

Difunctional (meth) acrylate monomers according to Formula I aretypically derived from diols containing an —(CH₂)— backbone.

Preferred compounds according to Formula I are polyoxytetramethylenediacrylate (Blemmer ADT250); 1,9 nonanediol diacrylate; 1,6 hexanedioldiacrylate (SR238); 1,6 hexanediol dimethacrylate (SR239); 1,4butanediol diacrylate (SR213); 1,2 ethanediol dimethacrylate (SR206);1,4 butanediol dimethacrylate (SR214); ethoxylated 1,6 hexanedioldiacrylate (Miramer M202)

Difunctional (meth) acrylate monomers according to Formula II aretypically derived from diols containing a glycol ether backbone. The R′group in Formula II is preferably H or methyl.

Preferred compounds according to Formula II are dipropyleneglycoldiacrylate (DPGDA, SR508), diethylene glycol diacrylate (SR230),triethyleneglycol diacrylate (SR272), 1,3-butylene glycol diacrylate,1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate,diethylene glycol dimethacrylate, dipropylene glycol diacrylate,ethylene glycol dimethacrylate, tetraethylene glycol diacrylate,tetraethylene glycol dimethacrylate, triethylene glycol diacrylate,triethylene glycol dimethacrylate, tripropylene glycol diacrylate,tripropylene glycol diacrylate, and combinations thereof.

The amount of the difunctional acrylate monomer according to Formula Ior II is at least 1 wt %, preferably at least 5 wt %, more preferably atleast 7.5 wt %, relative to the total weight of the curable liquid.

Multi Functional (Meth)Acrylate Monomer

Multi functional (meth) acrylate monomers are preferably selected fromthe group of tri-, tetra- or penta-functional (meth) acrylate monomers.It has been observed that the hardness of the cured layer obtained fromthe curable liquid becomes too high when too much tri-, tetra- orpenta-functional (meth) acrylate monomer is present in the liquid. Ithas been observed that the maximum concentration of the tri-, tetra- orpenta-functional (meth) acrylate monomer to ensure a proper hardnessdepends on their functionality. Typically, the higher theirfunctionality, the lower their maximum allowable concentration to ensurea Shore A hardness below 90.

Preferably, the maximum concentration of the tri-, tetra- orpenta-functional (meth)acrylate monomer, dependent on their viscosity,is as depicted in Table 1.

TABLE 1 visco (mPa · s) functionality <100 100-250 250-5000 >5000 3 20wt. % 17.5 wt. % 15 wt. %  10 wt. % 4 15 wt. % 12.5 wt. % 10 wt. % 7.5wt. % 5 10 wt. % ‘ 8 wt. %  6 wt. %   4 wt. %

For the curable aerosol jet liquid, the higher viscosities are allowableas described above. Therefore, higher concentrations of multifunctional(meth) acrylate monomers may be used.

Preferred examples are ditrimethylol propane tetraacrylate (DTMPTA),glycerol triacrylate and their alkoxylated, i.e. ethoxylated orpropoxylated, derivatives.

Specific compounds are trimethylol propane tetraacrylate (TMPTA),commercially available as Miramer M300; propoxylated TMPTA, commerciallyavailable as SR492; ethoxylated TMPTA, commercially available as MiramerM3130; DTMPTA, commercially available as SR355; propoxylated glyceryltriacrylate, commercially available as SR9021 and SR9020.

Other specific compounds are dipentaerythritol pentaacrylate (DIPEPA),commercially available as SR399LV; triacrylate esters ofpentaerythritol, such as pentaerythritol triacrylate (PETIA);tetra-acrylate esters of pentaerythritol, such as PETRA, commerciallyavailable as SR295; ethoxylated PETRA, commercially available as SR494;alkoxylated PETRA, commercially available as Ebecryl 40.

Urethane Acrylate Oligomer

The curable liquid may further contain monofunctional urethane acrylateoligomers. Urethane acrylates oligomers are well known and are preparedby reacting polyisocyanates with hydroxyl alkyl acrylates, usually inthe presence of a polyol compound. Their functionality (i.e. number ofacrylate groups) varies from 1 to 6. A lower functionality results inlower reactivity, better flexibility and a lower viscosity. The polyolcompound forms the backbone of the urethane acrylate. Typically thepolyol compounds are polyether or polyester compounds with afunctionality (hydroxyl groups) ranging from two to four. Polyetherurethane acrylates are generally more flexible, provide lower cost, andhave a slightly lower viscosity and are therefore preferred.

Commercially available urethane (meth)acrylates are e.g. CN9170,CN910A70, CN966H90, CN962, CN965, CN9290 and CN981 from SARTOMER;BR-3741B, BR-403, BR-7432, BR-7432G, BR-3042, BR-3071 from BOMARSPECIALTIES CO.; NK Oligo U-15HA from SHIN-NAKAMURA CHEMICAL CO. Ltd.;Actilane 200, Actilane SP061, Actilane 276, Actilane SP063 fromAKZO-NOBEL; Ebecryl 8462, Ebecryl 270, Ebecryl 8200, Ebecryl 285,Ebecryl 4858, Ebecryl 210, Ebecryl 220, Ebecryl 1039, Ebecryl 1259 andIRR160 from CYTEC; Genomer 1122 and Genomer 4215 from RAHN A.G. andVERBATIM HR50 an urethane acrylate containing liquid photopolymer fromCHEMENCE.

The curable liquid preferably comprises monofunctional urethane acrylateoligomers, more preferably monofunctional aliphatic urethane acrylates,having a very low viscosity of 100 mPa·s or lower at 25° C., like forexample Genomer 1122 (2-acrylic acid 2-{[(butylamino) carbonyl]oxy}ethylester, available from Rahn AG) and Ebecryl 1039 (available from CytecIndustries Inc.). The total amount of the monofunctional urethaneacrylate oligomer is preferably at least 5 wt %, more preferably atleast 7.5 wt %, relative to the total weight of the curable liquid.

The radiation curable liquid comprises an initiator which, upon exposureto radiation initiates the curing, i.e. polymerization, of the appliedliquid.

However, it is also possible to carry out the curing by electron beamradiation where the presence of an initiator is not mandatory.

Preferably a photo-initiator is used which upon absorption of actinicradiation, preferably UV-radiation, forms high-energy species (forexample radicals) inducing polymerization and cross linking of themonomers and oligomers of the deposited liquid.

A combination of two or more photo-initiators may be used. Aphoto-initiator system, comprising a photo-initiator and a co-initiator,may also be used. A suitable photo-initiator system comprises aphoto-initiator, which upon absorption of actinic radiation forms freeradicals by hydrogen abstraction or electron extraction from a secondcompound, the co-initiator. The co-initiator becomes the actualinitiating free radical.

Irradiation with actinic radiation may be realized in two steps, eachstep using actinic radiation having a different wavelength and/orintensity. In such cases it is preferred to use 2 types ofphoto-initiators, chosen in function of the different actinic radiationused.

Suitable photo-initiators are disclosed in EP-A 1637926 paragraph [0077]to [0079].

To avoid extraction of the photo-initiator out of the cured back layer,copolymerizable photo-initiators (and/or co-initiators) such asdisclosed in WO2012/084811 may be used. A preferred total amount ofinitiator is 1 to 10 wt %, more preferably 2.5 to 7.5 wt %, of the totalcurable liquid weight.

D. Phase Change Liquids

Phase change liquids are suitable for the invention since they solidifyafter jetting when contacting the support. They may comprise a lowmelting wax, and/or a gelling agent.

The low melting wax can comprise a polyalkylene wax, such as apolyethylene wax, a polypropylene wax, mixtures thereof, or the like.The polyalkylene wax(es) is present in any desired or effective amount.Examples of suitable polyalkylene waxes are disclosed in US 2007119340 Aparagraph [0024]. The low melting wax component can also comprisefunctional wax(es). In one embodiment functional alcohol waxes can beemployed herein. In a further embodiment the functional alcohol wax canbe a mono-functional alcohol wax, a di-functional alcohol wax, atri-functional alcohol wax, a tetra-functional alcohol wax, or mixturesthereof. Examples of suitable functional waxes are disclosed in US2007119340 A paragraph [0027].

The phase change liquid can also include a gelling agent. The gellingagent can comprise a crystalline or semi-crystalline gelling agent. Apreferred gelling agent is an ester-terminated amide. Theester-terminated amide is a semi-crystalline gelling agent that forms aclear gel. Examples of suitable ester-amide compounds and thepreparation thereof are disclosed in, for example U.S. Pat. No.5,863,319. Suitable ester-amides are also commercially available as, forexample, UNI-REZ® 2980 and UNICLEAR® 100 (commercially available fromArizona Chemical), and the like. In one specific embodiment the ureagelling agent is a daily urea material. Specific examples of suitableurea gelling agents are disclosed in U.S. Pat. No. 5,783,657 paragraph[0032].

In order to obtain phase-change inks which can be jetted at temperatureslower than conventional jetting temperatures which exhibit robustness,that is resistance to scratch, crease and abrasion with substantially nosmear, a colloidal dispersion of at least one of silica nanoparticlesand metal oxide nanoparticles can be combined with the low melting waxand the gelling agent. Examples of suitable colloidal dispersions aredisclosed in U.S. Pat. No. 5,783,657 paragraph [0018] and [0022].

In order to speed up the curing/solidification of the liquid uponcontact with the surface of the back side of the printing plateprecursor, the support can be cooled.

E. Radiation-Curable Phase-Change Liquids

Radiation curable phase change liquids comprise a vehicle and a curablesolid including at least one acrylate reactive group or methacrylatereactive group.

Curable solids for use in the radiation curable compositions includeradiation curable materials that are solids at room temperature and haveone or more unsaturated functional groups therein, such as one or morealkene, alkyne, and acrylate or methacrylate reactive groups. Inembodiments the curable solids are low molecular weight curable solids.As used herein, the term low molecular weight refers to compounds havinga weight average molecular weight of about 500 Daltons or less, such asabout 150 to about 450 Daltons or from about 200 to about 400 Daltons.In embodiments, the curable solid may be an alkyl acrylate, arylacrylate, alkylaryl acrylate, aryl alkyl acrylate, alkyl methacrylate,aryl methacrylate, alkylaryl methacrylate, aryl alkyl methacrylate.Exemplary unsaturated monomers for use as curable solids are disclosedin US 20110074895 A paragraph [0033] and [0034].

The vehicle including a curable wax, a gellant, at least onephotoinitiator, and optionally one or more co-monomers. The combinationof the co-monomers may aid in solubilising the gellant material. Theco-monomers may be chosen from any suitable radiation curable monomers.Examples of suitable curable monomers are disclosed in US 20110074895 Aparagraph [0040], [0041] and [0042].

The vehicle of the radiation-curable phase-change liquid may include atleast one gellant. The gellant functions to dramatically increase theviscosity of the vehicle and liquid within a desired temperature range.In particular, the gellant forms a semi-solid gel in the ink vehicle attemperatures below the specific temperature at which the ink compositionis jetted. In particular, jetted radiation-curable phase-change liquiddrops would be pinned into position on the support, that is at atemperature cooler than the ink-jetting temperature of the inkcomposition through the action of a phase-change transition in which theink composition undergoes a significant viscosity change from a liquidstate to a gel state (or semi-solid state). Gellants suitable for useare comprised of a curable amide, a curable polyamide-epoxy acrylatecomponent and a polyamide component, a curable composite gellantcomprised of a curable epoxy resin and a polyamide resin, mixturesthereof and the like, as disclosed in US 2010/304040 A. The curablegellant may also participate in the curing of monomer(s) of thecomposition. Amide gellants suitable for use include those described inUS 2008/0122914, U.S. Pat. No. 7,276,614 B and U.S. Pat. No. 7,276,614B.

The radiation-curable phase-change liquid preferably may include aninitiator, such as, for example, a photoinitiator for assisting incuring the ink. A photoinitiator that absorbs radiation, for example UVlight radiation, to initiate curing of the curable components of the inkmay be used. Examples are disclosed in US 20110074895 A paragraph[0089].

To the above described liquids, additives such as plasticizers,inhibitors, oxygen inhibitors, elastomeric binders, surfactants,colorants, solvents, humectants and biocides can be added.

Plasticizer

Plasticizers may be added to the curable liquid to increase the softnessand flexibility of the back layer and reduce the risk of mechanicallydamaging the image recording layer. Suitable plasticizers are benzylphthalates, triaryl phosphate esters, pentaerythritol tetrabenzoate,dialkyl adipate, dialkyl phthalates, dialkyl sebacate, alkyl benzylphthalates, ethylene glycol monostearate, glycerol monostearate,propylene glycol monostearate, dicyclohexyl phthalate, diphenylisophthalate, triphenyl phosphate, dimethyl isophthalate, and mixturesthereof. Specific examples of commercially available plasticizers aredisclosed in U.S. Pat. No. 7,276,614 B paragraph [0035] and the onedisclosed in EP 1637926 A ([0085]-[0091]). However such plasticizers maymigrate to the surface of the back layer and come into contact with theradiation sensitive layer. For that reason, it is preferred to use acopolymerizable plasticizing monomer such as a low Tg monomer of whichthe corresponding homopolymer has a glass transition temperature below−15° C. or diallylphthalate, as disclosed in EP 2466380 A.

Inhibitors

Suitable polymerization inhibitors can be added to the curable liquidsto increase the shelf life of the liquids. Preferable inhibitors includephenol type antioxidants, hindered amine light stabilizers, phosphortype antioxidants, hydroquinone monomethyl ether commonly used in(meth)acrylate monomers, and hydroquinone, methylhydroquinone,t-butylcatechol, pyrogallol may be used. Of these, a phenol compoundhaving a double bond in molecules derived from acrylic acid isparticularly preferred due to its having a polymerization-restrainingeffect even when heated in a closed, oxygen-free environment. Suitableinhibitors are, for example, Sumilizer® GA-80, Sumilizer® GM andSumilizer® GS produced by Sumitomo Chemical Co., Ltd. The amount of apolymerization inhibitor is generally between 200 and 20 000 ppm of thetotal curable liquid.

Oxygen Inhibitors

The radiation curable liquids which are based on addition polymerisationmay contain compounds which decrease the polymerization inhibition ofoxygen by means of radical polymerization inhibitors such as:2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butane-1 and1-hydroxy-cyclohexyl-phenyl-ketone; 1-hydroxy-cyclohexyl-phenyl-ketoneand benzophenone;2-methyl-1[4-(methylthio)phenyl]-2-morpholino-propane-1-on anddiethylthioxanthone or isopropylthioxanthone; and benzophenone andacrylate derivatives having a tertiary amino group, and addition oftertiary amines. An amine compound may commonly be employed to decreasean oxygen polymerization inhibition or to increase sensitivity. However,when an amine compound is used in combination with a high acid valuecompound, the storage stability at high temperature tends to bedecreased.

Synergist additives may be used to improve the curing quality and todiminish the influence of the oxygen inhibition. Such additives include,but are not limited to ACTILANE® 800 and ACTILANE® 725 available fromAKZO NOBEL, Ebecryl® P115 and Ebecryl® 350 available from UCB CHEMICALSand CD 1012, Craynor CN 386 (amine modified acrylate) and Craynor CN 501(amine modified ethoxylated trimethylolpropane triacrylate) availablefrom CRAY VALLEY. The content of the synergist additive is in the rangeof 0 to 20 wt %, preferably in the range of 5 to 15 wt %, based on thetotal weight of the curable liquid.

Elastomeric Binders

Elastomeric binders may also be comprised in the curable liquids toincrease flexibility and softness of the dried or cured layer. It may bea single binder or a mixture of various binders. The elastomeric bindercan be an elastomeric copolymer of a conjugated diene-type monomer and apolyene monomer having at least two non-conjugated double bonds, or anelastomeric copolymer of a conjugated diene-type monomer, a polyenemonomer having at least two non-conjugated double bonds and a vinylmonomer copolymerizable with these monomers. Preferred elastomericbinders are disclosed in EP 1637926 A paragraph [0092] and [0093].

Due to their high molecular weight, the addition of elastomeric bindersmay cause an increase in viscosity of the curable liquid. Therefore, theamount of elastomeric binder is preferably less than 5 wt % for theliquids which are applied via inkjet. As viscosity is not an issue, moreelastomeric binder, preferably more than 5 wt %, more preferably morethan 10 wt %, may be used for liquids which are used with the aerosoljet technique.

Surfactants

Surfactant(s) may be added to improve the coatability, jettability andspreading on the support of the liquid. The surfactants may be anionic,cationic, non-ionic, or zwitter-ionic and are usually added in a totalamount below 20 wt %, more preferably in a total amount below 10 wt %,each based on the total curable liquid weight.

Fluorinated or silicone compounds are preferably used as a surfactant,however, a potential drawback is bleed-out after image formation becausethe surfactant does not cross-link. It is therefore preferred to use acopolymerizable monomer having surface-active effects, for example,silicone-modified acrylates, silicone modified methacrylates,fluorinated acrylates, and fluorinated methacrylates.

Colorants

The curable liquids may comprise colorants to e.g. improve thevisibility of the back layer. The colorants may be dyes or pigments or acombination thereof. Organic and/or inorganic pigments may be used.Suitable dyes include direct dyes, acidic dyes, basic dyes and reactivedyes. Suitable pigments are disclosed in EP 1637926 A paragraphs [0098]to [0100]. The pigment may be present in the range of 0.01 to 10 wt %,preferably in the range of 0.1 to 5 wt %, each based on the total weightof curable liquid.

Solvents

The curable liquids preferably do not contain an evaporable component,but sometimes, it can be advantageous to incorporate an extremely smallamount of a solvent to improve adhesion to the support or reduce theviscosity of the liquid. In this case, the added solvent may be anyamount in the range of 0.1 to 10.0 wt %, preferably in the range of 0.1to 5.0 wt %, each based on the total weight of curable liquid.

Humectants

When a solvent is used in the curable liquids of the invention, ahumectant may be added to prevent the clogging of the nozzle, due to itsability to slow down the evaporation rate of curable liquid. Suitablehumectants are disclosed in EP 1637926 A paragraph [0105]. A humectantis preferably added to the curable jettable liquid formulation in anamount of 0.01 to 20 wt % of the formulation, more preferably in anamount of 0.1 to 10 wt % of the formulation.

Biocides

Suitable biocides to be used in combination with the described curablefluid include sodium dihydroacetate, 2-phenoxyethanol, sodium benzoate,sodium pyridinethion-1-oxide, ethyl p-hydroxy-benzoate and1,2-benzisothiazolin-3-one and salts thereof. A preferred biocide isProxel® GXL available from ZENECA COLOURS. A biocide is preferably addedin an amount of 0.001 to 3 wt %, more preferably in an amount of 0.01 to1.00 wt %, each based on the total weight of the curable liquid.

The Image Recording Layer

Suitable image recording layers of lithographic printing plateprecursors have been extensively described in the patent literature andcan be divided in negative working radiation sensitive and positiveworking sensitive layers.

Negative working plate precursors typically form an image by light- orheat-induced chemical cross linking or polymerisation of a photopolymercoating or by physical insolubilization due to heat-induced coalescence,fusing or melting of thermoplastic polymer particles. Specially designednegative plate precursor allow processing without hazardous developer,i.e. of high pH or containing a large amount of organic solvents, e.g.by using gums or a fountain solution of neutral or low pH. More detailsabout the composition of and methods of making negative workingradiation sensitive layers which are suitable examples are described ine.g. U.S. Pat. No. 4,378,564, U.S. Pat. No. 4,378,564, U.S. Pat. No.4,378,564, EP 1349006 A, EP 1614538 A, EP 0931647 A, WO 02/21215 and EP1817166 A.

In positive working plate precursors, the higher dissolution ofnon-printing areas is typically due to a kinetic differentiation of thedissolution process: the exposed areas dissolve more quickly in thedeveloper than the non-exposed areas, so that a lithographic image isobtained after a typical development time of 15 to 30 seconds. When atypical positive plate precursor is treated with a developer for severalminutes, both the exposed and the non-exposed areas dissolve in thedeveloper and no image is formed. More details about the composition ofand methods of making positive working radiation sensitive layers whichare suitable examples are described in e.g. US 2009/0197206 A, EP 823327A, WO 97/39894, EP 864420 A, WO 99/63407, EP 1826001 A, EP 901902 A, EP909657 A and EP 1159133 A.

Also materials for making a lithographic printing master by image-wisejetting of an “ink”, e.g. a curable liquid as described above, on alithographic support benefit from the present invention. The inkreceiving layer which is typically present in such materials, shall beregarded as the “image recording layer” in the meaning of the presentinvention. The ink receiving layer can be the surface of an anodisedaluminium support optionally treated with a post-anodic treatment asdescribed below. Another example of a suitable ink receiving layer isdisclosed in WO 01/34394.

The Support

A particularly preferred support as part of the invention is a grainedand anodized aluminium support. The aluminium support has usually athickness of about 0.1-0.6 mm. However, this thickness can be changedappropriately depending on the size of the printing plate used and/orthe size of the plate-setters on which the printing plate precursors areexposed. The Al support which can be used in the present invention has athickness preferably between 0.1 mm and 0.4 mm, more preferably between0.14 mm and 0.3 mm most preferably between 0.14 mm and 0.24 mm. Thealuminium is preferably grained by electrochemical graining ormechanical treatment, and anodized by means of anodizing techniquesemploying phosphoric acid or a sulphuric acid/phosphoric acid mixture.Methods of both graining and anodization of aluminium are well known inthe art.

By graining (or roughening) the aluminium support, both the adhesion ofthe printing image and the wetting characteristics of the non-imageareas are improved. By varying the type and/or concentration of theelectrolyte and the applied voltage in the graining step, different typeof grains can be obtained. The surface roughness is often expressed asarithmetical mean centre-line roughness Ra (ISO 4287/1 or DIN 4762) andmay vary between 0.05 and 1.5 μm. The aluminium substrate of the currentinvention has preferably a Ra value between 0.30 μm and 0.60 μm, morepreferably between 0.35 μm and 0.55 μm and most preferably between 0.40μm and 0.50 μm. The lower limit of the Ra value is preferably about 0.1μm. More details concerning the preferred Ra values of the surface ofthe grained and anodized aluminium support are described in EP 1356926A.

By anodising the aluminium support, its abrasion resistance andhydrophilic nature are improved. The microstructure as well as thethickness of the Al₂O₃ layer is determined by the anodising step, theanodic weight (g/m² Al₂O₃ formed on the aluminium surface) variesbetween 1 and 8 g/m². The anodic weight is preferably between 1.5 g/m²and 5.0 g/m², more preferably 2.5 g/m² and 4.0 g/m² and most preferably2.5 g/m² and 3.5 g/m².

The grained and anodized aluminium support may be subject to a so-calledpost-anodic treatment to improve the hydrophilic character of itssurface. For example, the aluminium support may be silicated by treatingits surface with a solution including one or more alkali metal silicatecompound(s)—such as for example a solution including an alkali metalphosphosilicate, orthosilicate, metasilicate, hydrosilicate,polysilicate or pyrosilicate—at elevated temperature, e.g. 95° C.Alternatively, a phosphate treatment may be applied which involvestreating the aluminium oxide surface with a phosphate solution that mayfurther contain an inorganic fluoride. Further, the aluminium oxidesurface may be rinsed with a citric acid or citrate solution, gluconicacid, or tartaric acid. This treatment may be carried out at roomtemperature or may be carried out at a slightly elevated temperature ofabout 30 to 50° C. A further interesting treatment involves rinsing thealuminium oxide surface with a bicarbonate solution. Still further, thealuminium oxide surface may be treated with polyvinylphosphonic acid,polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinylalcohol, polyvinylsulphonic acid, polyvinylbenzenesulphonic acid,sulphuric acid esters of polyvinyl alcohol, and acetals of polyvinylalcohols formed by reaction with a sulphonated aliphatic aldehyde,polyacrylic acid or derivates such as GLASCOL E15™ commerciallyavailable from Ciba Speciality Chemicals. One or more of these posttreatments may be carried out alone or in combination. More detaileddescriptions of these treatments are given in GB 1084070 A, DE 4423140A, DE 4417907 A, WO 01/54915, WO 00/46029, DE 4001466 A, EP 292801 A, EP291760 A and U.S. Pat. No. 4,458,005.

In a preferred embodiment, the support is first treated with an aqueoussolution including one or more silicate compound(s) as described abovefollowed by the treatment of the support with an aqueous solutionincluding a compound having a carboxylic acid group and/or a phosphonicacid group, or their salts. Particularly preferred silicate compoundsare sodium or potassium orthosilicate and sodium or potassiummetasilicate. Suitable examples of a compound with a carboxylic acidgroup and/or a phosphonic acid group and/or an ester or a salt thereofare polymers such as polyvinylphosphonic acid, polyvinylmethylphosphonicacid, phosphoric acid esters of polyvinyl alcohol, polyacrylic acid,polymethacrylic acid and a copolymer of acrylic acid and vinylphosphonicacid. A solution comprising polyvinylphosphonic acid or poly (meth)acrylic acid is highly preferred.

The support can also be a flexible support, which may be provided with ahydrophilic layer, hereinafter called ‘base layer’. The flexible supportis e.g. paper, plastic film or aluminium. Preferred examples of plasticfilm are polyethylene terephthalate film, polyethylene naphthalate film,cellulose acetate film, polystyrene film, polycarbonate film, etc. Theplastic film support may be opaque or transparent. The base layer may bepreferably a cross-linked hydrophilic layer obtained from a hydrophilicbinder cross-linked with a hardening agent such as formaldehyde,glyoxal, polyisocyanate or a hydrolyzed tetra-alkylorthosilicate. Thelatter is particularly preferred. The thickness of the hydrophilic baselayer may vary in the range of 0.2 to 25 μm and is preferably 1 to 10μm. More details of preferred embodiments of the base layer can be foundin e.g. EP 1025992 A.

A method of the present invention leads to supports of lithographicprinting plate precursors having an additional advantage of a reducedweight and cost. Indeed, by depositing a back layer, according to apreferred embodiment of the invention, the thickness of the support canbe reduced by a value equal to the thickness of the back layer while thetotal thickness of the support, being the sum in thickness of thesupport and the back layer, remains constant. The thickness of thesupport of a lithographic printing plate is mostly determined bydimensional requirements related to the used printing press, regardlesshow that thickness of the support is obtained. Using the back layerallows to obtain a required total thickness of the support of alithographic printing plate precursor with a thinner support. When usingAluminium as support, one of the advantages of using a less thickAluminium support is that cracking of the support due to mechanicalstress during high run-length printing jobs is less early to occur.Furthermore, for supports made of Aluminium, the reduction of thethickness of the support can represent a considerable reduction inweight and cost. Using back layers with a surface coverage preferablylower than 10%, more preferably lower than 5%, makes it possible toachieve a required thickness of the back layer without depositing toomuch liquid compare to a continuous back layer. The reduced amount ofliquid to be deposited at this low surface coverage of the back layerfurther reduces the cost of the support of the printing plate precursor.

Preferred embodiments of the invention include methods of depositingnon-continuous back layers with a valve-jet on an Al support having athickness of 0.14 mm. The liquid has a viscosity of 100 cPs or more andis cured after jetting by UV-LEDs. After jetting and curing, the backlayer consists of dots placed according to a regular pattern. The backlayer has an average thickness between 40 μm and 60 μm, and a surfacecoverage between 1 and 5% with a distance between the dots of between1.0 and 2.0 mm. After depositing the back layer, an image recordinglayer is coated on the front side of the support. Another preferredembodiment of the invention includes methods to deposit dots with avalve-jet on an Al support of 0.24 mm. The liquid has a viscosity of 100cPs or more and is cured after jetting by UV-LEDs. The dots are placedaccording to a regular pattern and after jetting and curing, the backlayer has an average thickness between 40 μm and 60 μm, a surfacecoverage between 1 and 5% and a distance between the dots of between 1.0and 2.0 mm. After depositing the back layer, an image recording layer iscoated on the front side of the support. Other preferred embodiments ofthe invention include methods of depositing a non-continuous back layeron an Al support with a thickness of 0.24 mm. The liquid is deposited bya valve-jet and the liquid has a viscosity of 100 cPs or more. Theliquid is cured by LEDs and after curing, the back layer has a shore Ahardness between 80 and 90. The back layer consists of dots placedaccording to a regular pattern and has an average thickness between 40μm and 60 μm, a surface coverage between 1 and 5% with a distancebetween the dots of between 1.0 and 2.0 mm. After depositing the backlayer, an image recording layer is coated on the front side of thesupport.

EXAMPLES

While the present invention will hereinafter be described in connectionwith preferred embodiments thereof, it will be understood that it is notintended to limit the invention to those embodiments.

Preparation of the Curable Liquids

Liquid 1 was prepared by adding the ingredients listed in Table 2 into arecipient. Each ingredient is added after the previous one is completelydissolved.

TABLE 2 Ingredient Amount (wt. %) Laromer TBCH 22.9 Miramer M202 13.9Mixture 1 9.2 Sartomer SR340 13.9 SR531 13.2 Sartomer CD 278 2.3Irgacure 819 11.5 Lucirin TPO L 0.7 EFKA 3600N 6.0 SR9035 6.0 SR339C 0.4

Liquid 2 was prepared by adding the ingredients listed in Table 3 into arecipient. Each ingredient is added after the previous one is completelydissolved.

TABLE 3 Ingredient Amount (wt. %) Miramer M202 5.00 SR339C 54.5 ACMO10.00 SR531 14.30 Mixture 1 0.70 Sartomer CD278 2.70 Sartomer SR90355.00 IRGACURE 819 3.50 Omnirad TPO-L 3.5 BYK UV 3510 0.4

All materials used in the examples were readily available from standardsources such as Aldrich Chemical Co. (Belgium) and Acros (Belgium)unless otherwise specified.

-   -   Laromer TBCH is a 4-t.butyl cyclohexyl acrylate from BASF    -   Miramer M202 is a 1,6 hexanediol (ethoxylated) diacrylate from        MIWON.    -   Mixture 1 is a mixture of 4 wt % p-methoxyphenol, 10 wt %        2,6-di-tert-butyl-4-methylfenol and 3.6 wt % Aluminium        N-nitroso-phenylhydroxylamine (available from CUPFERRON AL) in        dipropylene glycol diacrylate.    -   Sartomer SR340, a 2-phenoxyethyl methacrylate from SARTOMER.    -   SR531, a cyclic trimethylolpropane formal acrylate from        SARTOMER.    -   Sartomer CD 278, a monofunctional acrylate ester from SARTOMER.    -   ACMO is an acryloyl morpholine from RAHN AG    -   Irgacure 819 is a UV-photoinitiator from CIBA.    -   Lucirin TPO L, a UV-photoinitiator from BASF    -   Omnirad TPO L, a UV-photoinitiator from I G M RESINS BV    -   EFKA 3600N, a levelling agent from BASF    -   BYK UV 3510, polyether modified polydimethylsiloxane wetting        agent from BYK CHEMIE GMBH    -   SR9035, ethoxylated (15) trimethylolpropane triacrylate from        SARTOMER    -   SR339C, a 2-phenoxyethyl acrylate from SARTOMER

Application of the Back Layer According to a Predefined Image

A non-continuous back layer was applied as follows: a printing plateprecursor with a size of 30 cm×5 cm was fixed on a drum with the imagerecording layer in contact with the drum surface. On top of the drum, aprinting station with a fixed ink jet head (CA4 print head from ToshibaTec Corp.) was placed. The head was set in multidrop mode (8 drops perdot) to obtain drop volumes of 42 pl, the head driving voltage was 23 Vand the head temperature was 45° C. The drum speed was 300 m/s, theresolution 150 dpi and the firing frequency 24.8 kHz. The jettedpattern, consisting of dots according a predefined image was instantlycured with an LED bar (at 100% LED-output 395 nm, the LEDs used were the2UVM124 type LEDs from Baldwin Corp.) positioned directly after the inkjet head. After all the layers were jetted, the samples wereadditionally cured by rotating the drum for 1 minute at 100% output ofthe LEDs. The hardness of the obtained back layer, measured by means ofthe Durometer hardness test, was 84 Shore A for the back layer based onLiquid 1 and 67 Shore A for the back layer based on Liquid 2.

The predefined image was a 1-bit tiff file of 295 pixels wide and 1771pixels long with a resolution of 150 dpi. The screen cell is made of 16by 16 pixels. A 2 percent surface coverage corresponds with a 2 by 2pixel dot, a 25 percent surface coverage with an 8 by 8 pixel dot, andso on. The dots were placed according to a regular pattern.

Protection Against Scratching Example 1

The back side of TPNG-TP plates (Agfa Graphics), was printed accordingto a method described above, wherein the print head was jetting dropletsof Liquid 1. The predefined images contained regular dot patterns havingaverage surface coverage values as listed in Table 4. The samples werepre-conditioned during 4 hours at 23° C. and a RH of 50%. Two plateswere put together in such a way that the image recording layer was incontact with the back side of the adjacent plate. The two plates wherethen shifted one against the other at a speed of 30 mm/s. On top of theoverlying plate a weight of 20 g was put to simulate the pressure duringtransport of a stack of plates. One couple of plates were shifted oneagainst the other with an interleaf sheet (Pfleiderer Coral T2) inbetween, as a reference. The samples were then processed in an Elantrix85 processor (Agfa Graphics) wherein the developing section was filledwith ‘TP developer’ from Agfa Graphics. The developing conditions were adwell time of 25 s and a temperature of 27° C. The evaluation of thescratches in the image recording layer was based on the average width ofthe scratches. The results are summarised in Table 4.

TABLE 4 Number Average surface Width of scratches of jetted layerscoverage in % (μm)  0 0 Comp. 52 0 (with interleaf 0 Reference 0 sheet) 5 75 Inv. 31  5 50 Inv. 10  5 25 Inv. 21  5 2 Inv. 1 10 90 Inv. 0 10 75Inv. 0 10 50 Inv. 0 10 25 Inv. 0 10 2 Inv. 0 25 90 Inv. 0 25 75 Inv. 025 50 Inv. 0 25 25 Inv. 0 25 2 Inv. 0

From these results, it can be seen that with a back layer obtained byjetting drops of Liquid 1 in at least 10 layers on top of each other,the level of scratches of the image recording layer is equal to thelevel of scratches when an interleaf sheet is used between the back sideof the support and the image recording layer.

Example 2

The back side of ‘Elite Pro’ plates (Agfa Graphics), was printed asdescribed in example 1, using Liquid 1 and Liquid 2 as curable liquids.The test in which the plates were shifted one against the other wasidentical to the test in example 1. After the scratching, the sampleswere processed in an Elantrix 85 processor (Agfa Graphics) wherein thedeveloping section was filled with ‘Energy Elite Improved Developer’from Agfa Graphics. The developing conditions were a dwell time of 18 sand a temperature of 22° C. The evaluation of the scratches in imagerecording layer was done by visual inspection. 0: no damage of the imagerecording layer visible, 1: superficial damage of the image recordinglayer is visible, 2: shallow scratches in the image recording layer, 3:deep scratches in the image recording layer. The results are summarisedin Table 5.

TABLE 5 Number Average surface Scratch of jetted layers coverage in %Liquid level  0 Comp. 3 0 (with interleaf sheet) 0 Reference 2 15 2 Inv.1 0 15 10 Inv. 1 0 15 25 Inv. 1 1 15 50 Inv. 1 1 25 2 Inv. 1 1 25 10Inv. 1 1 25 25 Inv. 1 1 25 50 Inv. 1 1 15 2 Inv. 2 0 15 10 Inv. 2 2 1525 Inv. 2 1 15 50 Inv. 2 2 25 2 Inv. 2 0 25 10 Inv. 2 1 25 25 Inv. 2 025 50 Inv. 2 2

From these results, it can be seen that with a back layer obtained byjetting drops of Liquid 1 or 2 in at least 15 layers on top of eachother, the level of scratches of the image recording layer is equal orlower than the level of scratches when an interleaf sheet is usedbetween the back side of the support and the image recording layer.

Friction Coefficient Example 3

Changing the thickness and the surface coverage of the non-continuousback layer makes it possible to change the static and dynamic frictioncoefficient as can bee seen in Table 6. To prove this, the frictioncoefficient of the plates obtained in examples 1, with the imagerecording layer contacting the back side of the overlying plate, hasbeen measured. The measurement of the friction coefficient is doneaccording to the method ISO 8295:1995.

TABLE 6 Average surface Static friction Dynamic friction Number oflayers coverage in % coefficient coefficient 0 0 0.32 0.29 5 2 0.5 0.535 25 0.77 0.77 5 50 0.88 0.71 5 75 0.77 0.71 5 90 0.77 0.7 10 2 0.660.59 10 25 1.2 1.01 10 50 1.27 1.05 10 75 1.95 1.11 10 90 1.92 1.01 25 21.04 0.68 25 25 2.53 1.28 25 50 2.7 1.07 25 75 2.8 1.25 25 90 2.71 1.46

To reduce the creation of scuffs due to movement of the plates over eachother during transport, it is favourable that the friction coefficientis not too low. Monitoring the friction coefficient by means of thesurface coverage and/or thickness of the back layer is an importantadvantage of the invention.

Stickiness Example 4

The back side of N94 VCF plates (Agfa Graphics), was printed in the sameway as in example 1. The comparison sample (COMP-1) was obtained bycoating the back side of the N94 VCF plates (Agfa Graphics) having asize of 24 cm×44 cm with Liquid 1. The coating was done by a wiredcoating bar to obtain a back layer with a thickness of 15 μm aftercuring. The curing was done with UV-A light (light box equipped with 8Philips TL 20 W/10 UVA (λ_(max)=370 nm) lamps; the distance between thesample and the lamps was ±10 cm). The samples were stacked with theimage recording side against the back side of the overlying plate andthe stack was put at 30% of RH and 85% of RH, placed under a pressure of2500 kg for 10 minutes to simulate prolonged storage conditions of astack of plates in a warehouse. After this test, the plates wereun-stacked and the stickiness was evaluated. The degree of stickinesscorresponds with a ranking from 1 (no stickiness at all) to 4 (verysticky and transfer of deposited material from the back side to theimage recording layer side). The results are summarized in Table 7.

TABLE 7 Average surface coverage (%) 30% of RH 85% of RH 100 (COMP-1) 23  25 (INV.) 4 4  2 (INV.) 1 1

At low surface coverage, a non-continuous back layer shows a reducedstickiness with respect to a uniform coating on the back side.

Example 5

The back side of TPNG-TP plates (Agfa Graphics), was printed in the sameway as in example 4. The comparison sample (COMP-2) was obtained thesame way as in example 4. The samples were stacked in identicalconditions as in example 4. The sticking power was measured by pullingapart the sandwiches at an angle of 180° with the Instron 33R69 pullbank at a speed of 10 mm/min. The results are summarised in Table 8.

TABLE 8 Average surface coverage in % Sticking power 2 Inv. None 25 Inv.Too low to measure 30 Inv. Too low to measure 40 Inv. Too low to measure50 Inv. Too low to measure 100 COMP-2 510N

The results show that a non-continuous back layer, deposited accordingto a preferred embodiment of the invention shows a strong reduction instickiness of the plates in a stack compared to a back layer applied viaa coating technique which is not capable of depositing liquids accordingto a predefined image.

Example 6

Two aluminium plates, having a thickness of 0.140 mm and a thickness of0.190 mm, are grained and anodised the same way as described in [0098]of EP2366545A. Onto the front side of the two grained and anodisedaluminium supports a coating solution having the composition CS-01 asdescribed in EP2366545A was coated and dried as described in [0099-0100]of EP2366545A. The coated printing plate precursors were cut into plateshaving a length of 210 mm and a width of 22 mm. Onto the back side ofthe printing plate precursor having an aluminium support with athickness of 0.140 mm, a back layer was deposited according to themethod described in Example 1. The print head was jetting droplets ofLiquid 2 and 15 layers were jetted on top of each other. The thicknessof the back layer, measured from the surface of the support to the topof the dots was 50 μm. The predefined image contained a regular dotpattern as described above and the surface coverage of the back layer is2%. The axis of the regular dot pattern which orientation is the closestwith the longest edge of the plate, shows an angle of 15° with the edgeof the printing precursor plate. The total thickness of the supportbeing the sum of the aluminium support and the back layer is hence:0.190 mm. This sample is called the inventive sample. The sample of theprinting plate precursor having an aluminium support of 0.190 mm ishereafter denoted as the comparative sample. The total thickness of thesupport of both samples is thus equal to 0.190 mm. The samples werepre-conditioned during 4 hours at 23° C. and a RH of 50%. Both sampleswere placed between 2 steel rollers having a diameter of 50 mm. Thesamples were bend mechanically with both ends toward the surface of oneroller and subsequently bend toward the surface of the second rollerwith a frequency of 20 bending movements per minute. The average numberof cycles at which cracking of the aluminium support occurred was forthe comparative sample 30.000 cycli and for the inventive plateprecursor sample 22.000 cycli. These results show that the durability ofthe lithographic printing plate with a back layer according to apreferred embodiment of the invention in long run length print jobs willbe higher than with a lithographic printing plate without a back layerfor the same total thickness of the support.

What is claimed is:
 1. A method for manufacturing a lithographicprinting plate precursor, the method comprising the steps of: providinga support as a web, the support including a front side and a back side;coating an image recording layer on the front side of the support; anddepositing a liquid on the back side of the support using a depositingtechnique selected from the group consisting of ink jet printing, valvejet printing, and aerosol jetting to obtain a non-continuous back layeraccording to a predefined image.
 2. The method according to claim 1,further comprising the step of: at least partially curing the liquid;wherein the at least partially cured liquid has a Shore A hardness of 90or less.
 3. The method according to claim 2, wherein the liquid includesa (meth)acrylate monomer.
 4. The method according to claim 3, whereinthe (meth)acrylate monomer is a monofunctional (meth)acrylate monomer ora difunctional (meth)acrylate monomer.
 5. The method according to claim3, wherein the (meth)acrylate monomer is a multifunctional(meth)acrylate monomer.
 6. The method according to claim 2, wherein theliquid is at least partially cured by ultra violet light or electronbeam radiation.
 7. The method according to claim 1, wherein the liquidis a phase-change liquid.
 8. The method according to claim 1, whereinthe predefined image includes a cutting scheme defining predefinedcutting edges of the lithographic printing plate precursor, the methodfurther comprising the step of: slicing and cutting the web along thepredefined cutting edges.
 9. The method according to claim 8, whereinthe web is sliced and cut in sections within 1 cm of the predefinedcutting edges.
 10. The method according to claim 8, wherein thepredefined image includes a regular pattern of dots, lines, voids, or acombination thereof; and the regular pattern does not coincide with thepredefined cutting edges.
 11. A lithographic printing plate precursorcomprising: a support; an image recording layer provided on a front sideof the support; and a non-continuous back layer provided on a back sideof the support; wherein the non-continuous back layer includes apredefined image and has a Shore A hardness of 90 or less.